'7  if  fc  Med  Slain  lm$ 


Issin  d 


U.S.  DEPARTMENT  OF   AGRICULTURE, 

BUREAU  OF  PLANT  INDUSTRY— Circular  No.  20. 
B.  T.  GALLOWAY,  Chief  <>f  Bureau. 


AN  ELECTRICAL  RESISTANCE  METHOD  FOE 

THE  RAPID  DETERMINATION  OF  THE 

MOISTURE  CONTENT  OF  CHAIN. 


LYMAN  J.  BRIGGS, 
Physicist  in  Charge  of  Physical  Laboratory. 


98     I  if.  20    08 


ASHINGTO*  :  GOVEHNMEWT  PRINTING  OFFICE  :  1*08 


BUREAU  OF  PLANT  INDUSTRY. 

Physiologist  and  Pathologist,  and  Chief  of  Bureau,  Beverly  T.  Galloway. 
Physiologist  and  Pathologist,  and  Assistant  Chief  of  Bureau,  Albert  F.  Woods. 
Laboratory  of  Plant  Pathology,  Erwin  F.  Smith.  Pathologist  in  Charge. 
Fruit  Disease  Investigations,  Merton  B.  Waite,  Pathologist  in  Charge. 
Laboratory  of  Forest  Pathology,  Haven  Metealf.  Pathologist  in  Charge. 

Cotton  and  Truck  Diseases  and  Plant  Disease  Survey,  William  A.  Orion.  Pathologist  in  Charge. 
Pathological  Collections  and  Inspection  Work,  Flora  W.  Patterson,  Mycologist  in  Charge. 
Plant  Life  History  Investigations,  Walter  T.  Swingle,  Physiologist  in  Charge. 

Cotton  Breeding  Investigations,  Archibald  D.  Shamel  and  Daniel  X.  Shoemaker,  Physiologists  in  Charge. 
Tobacco  Investigations,  Archibald  D.  Shamel,  Wightman  \V.  Garner,  and  Ernest  II.  Mathewson,  in  Charge. 
Corn  Investigations.  Charles  P.  Hartley,  Physiologist  in  Charge. 

Alkali  and  Drought  Resistant  Plant  Breeding  Investigations,  Thomas  IT.  Kearney,  Physiologist  in  I 
Soil  Bacteriology  and  Water  Purification  Investigations,  Karl  F.  Kellerman,  Physiologist  in  Chargi  . 
Bionomic  Investigations  of  Tropical  and  Subtropical  Plants,  Orator  F.  Cook,  Bionomist  in  Ch  irge. 
Drug  and  Poisonous  Plant  and  Tea  Culture  Investigations,  Rodney  II.  True,  Physiologist  in  Charge. 
Physical  Laboratory,  Lyman  J.  Briggs,  Physicist  in  Chai 

Crop  Technology  and  Fiber  Plant  Investigations,  Nathan  A.  Cobb,  Crop  Technologist  in  Charge. 
Taxonomic  and  Range  Investigations,  Frederick  V.  Coville,  Botanist  in  Charge. 
Farm  Management.  William  .1.  Spillman,  Agriculturist  in  Clin 
Grain  Investigations,  Mark  Alfred  Carleton,  Cerealist  in  Charge. 

Arlington  Fiperirnental  Farm  and  Horticultural  Investigations.  Lie  C.  Corbett,  Horticulturist  in  Charge. 
Vegetable  Testing  Gardens,  William  W.  Tracy,  sr..  Superintendent. 
Sugar-Beet  Investigations,  Charles  O.  Townsend,  Pathologist  in  Charge. 
Western  Agricultural  Extension.  Carl  S.  Scofleld,  Agriculturist  in  Charge. 
Dry-Land  Agriculture  Investigations,  E.  Channing  Chilcott.  Agriculturist  in  Charge. 
Pomological  Collections,  Gustavus  B.  Brackett,  Pomologist  in  Charge. 

Field  Investigations  in  Pomology.  William  A.  Taylor  and  G.  Harold  Powell,  Pomologists  in  Charge. 
Experimental  Gardens  and  Grounds,  Edward  M.  Byrnes,  Superintendent. 
Foreign  Seed  and  Plant  Introduction,  David  Fairchild,  Agricultural  Explorer  in  Char.'. 
Forage  Crop  Investigations,  Charles  V.  Piper,  Agrostologist  in  Charge, 
Seed  Laboratory.  Edgar  Rrown, •Botanist  in  Charge. 
Grain  Standardization.  John  D.  Shanahan,  Crop  Technologist  in  Chargi 
Subtropical  Garden.  Miami,  Fla..  P.  J.  Wesler.  in  Charge. 

Plant  Introduction  Garden,  Chico,  Cal.,  W.  W.  Tracy,  jr..  Assistant  Botanist  in  Charge. 
South  Teras  Garden,  Brownsville,  Tex.,  Edward  C.  Green,  Pomologist  in  Charge. 
Farmers'  Cooperative  Demonstration   Work.  Seaman   \.  ECnapp,  Special  Agent  in  Charge, 
Seed  Distribution  (directed  by  Chief  of  Bureau),  Lisle  Morrison.  Assistant  in  General  Charge. 


Editor,  J.  E.  Rockwell. 
Chief  Clerk.  James  I'..  Jones. 
[Cir.  20] 
2 


It.   I  .   [.—422. 

A.\  ELECTRICAL  RESISTANCE  METHOD  FOR  THE 
RAPID  DETERMINATION  OF  THE  MOISTURE 
CONTENT  OF  GRAIN. 


INTRODUCTION. 

The  shipping  and  storing  qualities  of  grain  arc  so  dependent  upon 
it-  moisture  contenl  that  an  accurate  knowledge  of  the  moisture 
in  grain  in  storage  and  in  transil  is  highly  desirable.  This  subject 
lias  been  given  special  attention  h\  Brown  and  Duvel,"  who  have 
described  a  rapid  method  of  making  such  moisture  determinations. 
Their  method  consists  in  boiling  the  -rain  in  an  oil  having  a  flashing 
poinl  much  above  the  boiling  poinl  of  water,  condensing  the  water 
which  distills  off,  and  collecting  and  measuring  it  in  a  suitable 
graduate.  Moisture  determinations  can  by  this  method  l>c  made  in 
about  one-half  hour,  whereas  determinations  in  the  water  oven 
require  several  days.  This  method  is,  however,  suitable  for  labora- 
tory use  only,  necessitating  the  collecting  of  samples  before  the 
determinations  can  be  made,  and  does  not  appear  to  be  adapted  to 
such  grain  products  as  meal  and  flour. 

Vi  the  request  of  the  Office  of  Grain  Standardization,  the  writer 
undertook  the  development  of  an  electrical  resistance  method  for 
measuring  the  moisture  content  of  grain  adapted  to  measurements 
in  a  car  or  elevator,  as  well  as  in  a  laboratory,  and  requiring 
only  two  or  three  minutes  for  a  determination.  The  measurements 
so  far  have  been  confined  to  wheat.  The  results  obtained  are  so 
promising  that  a  brief  preliminary  description  of  the  method  is 
given.  Corresponding  measurements  will  he  made  for  other -rain-, 
a-  well  a-  for  Hour  and  corn  meal.  A  portable  apparatus  suitable 
for  measurements  in  car-  and  elevators  i-  also  being  constructed. 

DESCRIPTION    OF    THE    ELECTRICAL    RESISTANCE    METHOD    FOR 
MEASURING  THE  MOISTURE  CONTENT  OF  GRAIN. 

Tic  method  developed  consists  essentially  in  the  measurement  of 
the  resistance  offered  to  the  passage  of  an  electric  current  through 
the  grain  from  one  metallic  rod  or  electrode  to  another.     The  el 


Bullel  .111  of  Plant  Industry .     1907. 

[fir.  .'II] 


4  DETERMINATION    OF    MOISTURE    CONTENT    OF    GRAIN. 

trical  resistance  decreases  rapidly  as  the  moisture  content  of  the 
grain  increases.  The  electrical  resistance  of  wheat  containing  13 
per  cent  of  moisture  is  seven  times  that  of  wheat  containing  14  per 
cent  and  fifty  times  that  of  wheat  containing  15  per  cent  of  moisture. 
This  method,  therefore,  gives  a  very  open  scale,  and  a  considerable 
variation  in  resistance  can  take  place  without  seriously  affecting 
the  accuracy  of  the  moisture  determinations. 

The  relation  between  the  electrical  resistance  and  the  moisture 
content  of  wheat  is  shown  graphically  in  figure  1.  The  moisture 
percentages  in  this  figure  are  plotted  as  ordinates  and  the  natural 
logarithms  of  the  corresponding  resistances  are  plotted  as  abscissas. 
Five  widely  differing  types  of  wheat — soft  red  winter,  hard  red  winter, 

Xo.  1  hard  spring, 
durum,  and  a  badly 
mixed  wheat  con- 
taining many  weed 
seeds — were  used  in 
these  determina- 
tions.  The  close- 
ness with  which  the 
different  points  on 
the  diagram  ap- 
proach the  straight 
line  drawn  through 
them  illustrates  the 
accuracy  with  which 
moisture  determi- 
nations can  be  made 
by  this  method. 
The  logarithms  of 
the  resistances  in- 
stead of  the  resistances  themselves  are  plotted  in  order  to  condense 
the  diagram  and  to  bring  out  the  straight  line  relation  between  the 
two  variables  as  shown. 


/e 

X 

r 

r 

1 

* 

« 

V? 

-OG- 


Fig.  1.— Chart  showing  the  relation  between  the  moisture  eontent  and 
the  electrical  resistance  of  wheat.  Measurements  made  at  75°  F. 
For  description  of  electrodes,  see  text.  Resistances  expressed  in 
megohms.    Moisture  percentages  based  on  weight  of  moist  grain. 


RELATION  OF  ELECTRICAL  RESISTANCE  TO  TEMPERATURE. 

The  electrical  resistance  of  wheat  is  also  dependent  upon  the  tem- 
perature of  the  grain.  In  fact,  the  rapidity  with  which  the  resist- 
ance decreases  as  the  temperature  increases  i>  quite  remarkable  and 
greatly  exceeds  lha!  occurring  in  most  substances.  The  manner  in 
which  the  electrical  resistance  of  wheat  varies  with  the  temperature 
is  shown  graphically  in  figure  2,  in  which  temperatures  are  plotted 
as  onlinates  and  electrical  resistances  as  abscissas.  The  resistance 
at  4°  C.  (39°  F.)  is  seen  to  be  IS  times  the  resistance  at  24°  C.  (75°  F.). 

LClr.  201 


DETERMINATION    OF    MOISTURE  CONTENT   0]     GRAIN.  5 

This  curve  is  based  upon  34  groups  of  measurements  made  upon 
hard  red  winter,  sofl  red  winter,  hard  red  spring,  durum,  and  a 
mixed  wheat.  Dots  on  the  diagram  refer  to  one  sample,  crosses  to 
another,  and  so  on.  In  order  to  construct  a  mean  temperature 
resistance  curve,  the  resistances  corresponding  to  the  differenl  sam- 
ples were  all  increased  or  decreased  tn   an  am i  corresponding  to 

the  mean  of  the  ratios  of  the  resistances  to  the  corresponding  resist- 
ances of  one  curve  taken  as  a  standard.  In  making  these  determi- 
nations, the  wheat,  after  being  cooled  in  an  ice  chest,  was  allowed 
to  approach  the  temperature  of  the  room  and  a  series  of  resistance 
measurements  were  made  as  the  temperature  increased.     The  grain 


^ 

l\.° 

f* 

* 

•  ^ 

• 

° 

--     Q 

1 — 

cW  fO  so  so  /oo  /?o  /to  /GO  /so  ?oo 

/=r£\5~/^~7--^/^scr£'—/w£r&o>-"^*s~ 

•  ol  temperature  upon  the  electrical  resistance  of  wheat. 


was  in  each  case  stirred  to  obtain  as  uniform  a  temperature  distri- 
bution as  possible  before  each  sel  of  measurements.  Temperatures 
above  thai  of  the  room  were  obtained  in  a  similar  manner  by  heating 
the  grain  and  measuring  the  resistance  as  ii  cooled.  It  is  difficult 
to  determine  the  true  temperature  of  grain  while  it  is  being  wanned 
or  cooled  in  this  way,  which  account-  for  the  father  wide  departure 
of  some  of  the  points  from  the  mean  curve. 

THE  DETERMINATION  OF  THE  MOISTURE  CONTENT  OF  WHEAT  AT 
DIFFERENT  TEMPERATURES. 

\\\  combining  the  data  shown  in  figures  1  and  2  we  can  construct 
a  charl  showing  the  moisture  content  of  a  -ample  of  wheat  corre- 
sponding to  a  given  electrical  resistance  at  anj    temperature  within 

leu- 


6  DETERMINATION    OF    MOISTURE    CONTENT   OF    GRAIN. 

the  range  of  the  experiments.  Such  a  chart  is  presented  as  figure  3. 
Tliis  chart  is  similar  to  that  shown  as  figure  1,  except  that  we  have 
here  lines  showing  the  relation  between  moisture  content  and  resist- 
ance not  only  for  a  single  temperature,  as  in  figure  1,  but  for  tempera- 
ture intervals  of  5  degrees  from  80°  to  40°  F.  In  this  chart  the 
moisture  contents  are  plotted  as  ordinates  and  the  logarithms  of  the 
electrical  resistances  as  abscissas.  To  facilitate  the  use  of  the  chart, 
resistances  are  written  in  place  of  the  corresponding  logarithms. 
To  illustrate  the  use  of  the  chart,  suppose  that  a  resistance  of  55 
megohms  was  observed  in  a  given  sample  of  wheat  at  a  temperature 
of  75°  F.  Referring  to  the  chart,  it  will  be  seen  that  the  imaginary 
line  corresponding  to  55  megohms  crosses  the  75°  F.  line  at  a  point 
corresponding  to  13.95  per  cent  of  moisture.  This  statement 
assumes,  of  course,  that  the  measurements  were  made  with  elec- 
trodes of  standard  size,  to  which  tins  chart  is  only  applicable. 

APPARATUS  FOR  MEASURING  ELECTRICAL  RESISTANCE  OF 

GRAIN. 

Unless  the  grain  is  very  wet,  its  specific  electrical  resistance  is 
very  high.  The  resistance,  while  electrolytic  in  character,  is  so 
great  that  polarization  is  not  troublesome  and  measurements  can 
be  made  with  direct  currents.  The  electrical  apparatus  required 
fur  such  measurements  is  therefore  similar  to  that  used  tor  testing 
the  insulation  of  cables.  The  measurements  described  were  made 
principally  with  a  Wheatstone  bridge,  using  a  fairly  sensitive  gal- 
vanometer and  an  electromotive  force  of  17  volts.  In  the  driest 
samples  (below  12  per  cent)  the  resistance  was  so  high  that  it  could 
not  be  measured  by  this  method.  For  these  samples  the  direct 
deflection  method  was  used,  the  galvanometer  and  grain  resistance 
being  connected  in  series  with  a  battery  having  an  electromotive 
force  of  10  volts. 

In  all  the  measurements  described,  the  electrodes  used  consisted 
of  two  parallel  l-inch  round  brass  rods,  lj  inches  between  centers 
and  12  inches  long.  These  rods  were  kepi  parallel  and  insulated 
from  each  other  by  being  supported  in  a  hard-rubber  block  at  their 
upper  ends.  Connecting  wires  with  extra  heavy  rubber  insula- 
tion were  soldered  to  the  two  upper  ends  of  the  electrodes.  The 
grain  during  measurements  was  held  in  glass  battery  jars  5  inches 
in  diameter  and  11  inches  high.  The  height  of  the  *grain,  inside 
measurement,  was  10  inches.  The  lower  ends  of  the  electrodes 
rested  upon  the  bottom  of  the  jar.  The  temperature  was  measured 
with  a  mercurial  thermometer  having  a  cylindrical  bulb,  which  could 
We  readily  forced  into  the  grain. 

[CIr.  20] 


DETERMINATION    OF    MOISTURE    CONTEN RAIN. 


°o     «fc     °o     °o       °S 

*      s     to      K       > 


/oooo 

9000 
8000 
7000 

sooo 
sooo 


^o 


so 


60 


70 


Fig.  3.    <  rminlng  them'  otent  of  wheat  when  the  electi  adtem- 

perature  are  known.    El  dimensions  [escribed  in  the  text  must 

« itli  this  (hurt. 
[Clr.  20] 


8  DETBEMINATION   OF  MOISTURE   CONTENT  OF  GKAIN.  ^ 

Before  each  measurement  the  electrodes  were  removed  and   the      | 
1  oaeked  by  iarring  the  bottom  of  the  container  agatnai  some    |j 
fohd  Zct      It         "portent  that  .his  precaution  in  packing  be    oj 
ob    r^Tf  satisfactory  results  are  to  be  obtained      Tins  wUl  not  be   ., 
X  in  measurements  made  in  cars    since  the  settlmg  of  the      . 
™w,  in  transit  will  hace  reduced  it  to  a  stable  conditwn.  »i 

g  O  1  er  t\me    oTelectrodes  and  containers  designed  for  usmg  smaller    |; 
^SX.  hace  been  tried,  but  the  most  sahsfac.ory  results    s. 

aeo  in  the  Division  of  Soils  for  the  measurement  of  the  moisture  con 
Zti  'frr t'    ittrot  rnot  encountered  m  the  me— 

not  been  indicated  in  the  samples  so  far  examined. 

SUMMARY. 

This  paper  deals  with  an  electrical  -f^.™^1^2S 
determination  of  the  moisture  content  ^-*  «  „f 
haye  so  far  been  co nfined  to  wheat^ The^te ct^  ^ 

wheat  contammg  13  per  cent  of  u ^   b<j 

containing   15  per  cent     The  ^^  todfeate    that    the 

determined.     The    results    of    th     ^P>™c ,  bable 

moisture  content  ean  be  determined  **■£££     ^    p 
„ror   not   exceeding   0.3  j«r    «  The       paratua  is 

■  J  S3Ea"^S  o^her  grained  gr products  U  now  being 

investigated. 

Approved: 

James  Wilson,         . 

Secretary  of  Agriculture- 

Washington,  D.  C,  October  2P,  J«W. 

|  (if.  20]  q 


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